DESCRIPTION: The objective of this study is to examine the correlation between the discharge of pallidal neurons and three other measures: 1) clinical motor signs; 2) metabolic activity of cortical and subcortical structures at rest (in the absence of voluntary movement); and 3) metabolic activity of the same structures during movement. Clinical motor signs will be measured preoperatively, during the pallidotomy operation, and 12 months postoperatively. Preoperative measures will include motor measures of the CAPIT protocol plus assessment of reaction time and time to complete a simple tracking class. In the operating room, bradykinesia will be assessed by timing hand movements made with the hand in a "data glove" and shoulder movements made with the arm in a manipulandum. Tremor will be assessed using an accelerometer attached to the tremulous limb. Rigidity will be assessed as the displacement produced by a torque perturbation of the wrist and shoulder. The hypotheses to be tested are: 1) the presence and severity of bradykinesia will be most closely related to increased mean discharge rate in GPi (and perhaps to an increased incidence and decreased specificity of responses to somatosensory stimulation), while tremor will be associated with an increased incidence of rhythmic bursting activity in GPi; 2) the presence and severity of rigidity will be correlated with increased mean pallidal discharge in areas different than those with high activity associated with bradykinesia; and 3) in patients who receive liquid Sinamet (l-DOPA) in an amount sufficient to produce an "ON" state during intraoperative recording, GPi neurons will have a decreased mean firing rate, a decreased incidence of bursting, and a reduction in the incidence and distribution of somatosensory receptive fields, compared to those who do not receive Sinamet, and these changes will correlate with the clinical symptomatology in the "ON" vs. the "OFF" state. The differences in neural activity recorded in the different conditions (patients who are akinetic/rigid or akinetic non-rigid, each studied either in the "OFF" condition, or in the Sinamet-induced "ON" condition) will be compared to the differences in cortical metabolic activity, as assessed with PET using the same behavioral tasks.